Oxygen-depolarized cathodes for electrolytic diaphragm cells are not new. Heise et al. in U.S. Pat. No. 2,273,795, Feb. 17, 1942, show a porous cathode wherein air or oxygen is blown through the cathode and into the catholyte.
Butler et al. in Canadian Patent No. 700,933, Dec. 29, 1964, show a hollow cathode such as a porous carbon cylinder which may be impregnated with a metal catalyst such as silver and through which air or oxygen is blown into the catholyte. In the catholyte are suspended particles or graphite, metal-coated graphite or metal which are kept suspended by air or oxygen blown through the cathode. They are said to act as collectors for the oxygen admitted through the cathode and react with the hydrogen evolved in the cathodic portion of the cell to form water.
Butler in U.S. Pat. No. 2,681,884, June 22, 1954, shows a porous carbon cathode into which air or oxygen can be passed, and shows the oxygen to react with water to form hydroxyl ions which are said thereafter to react with hydrogen ions in the vicinity of the active wall of the cathode.
Juda in U.S. Pat. No. 3,262,868, July 26, 1966, shows a fuel cell said to be useful also in electrolytic procedures. The cell comprises a porous catalytic fuel anode and a porous catalytic oxidant cathode, at least two ion exchange membranes between these electrodes disposed in a spaced relationship with each other and with both electrodes so as to define at least three compartments adapted to contain liquids therein, means for passing liquids into and out of these compartments, means for passing a catalytically reactive combustible fuel into the anode, means for passing a catalytically reactive oxidant gas into the cathode and means for passing a direct electric current between both electrodes.
Billiter in British Pat. No. 832,196, published Apr. 6, 1960, discloses preferably single metal cathodes in grid, wire mesh and sintered masses of small-sized metal grains and the like in the form of narrow boxes, hollow plates or the like. In use in electrolytic cells for electrolyzing aqueous solutions of alkali-metal compounds, air or oxygen under pressure is passed through such structures and may pass into the catholyte. Hydrogen evolution at the cathode is thereby said to be prevented.
Caesar in U.S. Pat. No. 3,377,265, Apr. 9, 1968, discloses a porous electrode for electrochemical cells comprising an electron-conductive support, a thin layer of fibrous alumina monohydrate on the support and an electrochemical catalyst disposed in and on the alumina layer. Perforate nickel may be the electro-conductive support. A carbon support having large pores may have disposed on its surface an alumina film having smaller pores to give a dual porosity electrode. In such electrolyte, the fuel gas makes contact on the side of the large pores and the aqueous electrode makes contact on the side of the small pores.
Gritzner in U.S. Pat. No. 3,923,628, Dec. 2, 1975, in the first of a series of four patents, discloses an oxygen-depolarizing cathode for use in a cell for producing chlorine and an alkali metal hydroxide from aqueous alkali metal chloride. The cathode shown has a wall portion adapted to be in contact with the catholyte and another wall portion substantially simultaneously adapted to be in contact with an oxidizing gas. A surface portion of the cathode at least partially defines an oxidizing gas compartment into which oxidizing gas, preferably oxygen, is fed. The cathode is advantageously a foraminous body having at least the surface including Ru, Rh, Pd, Ag, Os, Ir, Pt or Au with a coating of particulate metal admixed with a polymeric tetrafluoroethylene or a copolymer of hexafluoropropylene and tetrafluoropropylene. In one aspect, the cathode is a silver-coated woven copper screen with a mesh size of about 20 to about 50 with a coating of a mixture of platinum, silver or carbon particulates and a polymer or copolymer of the type indicated above. The oxygen prevents polarization by avoiding liberation of hydrogen at the wall in contact with the catholyte.
It is desired to provide an improved apparatus and process for reducing the electrical consumption of chlorine-producing electrolytic diaphragm cells.